1. Field of the Invention
The present invention relates to the field of power electronics. In particular it relates to a controllable power semiconductor component, comprising
(a) an anode, a cathode and a gate; PA0 (b) between the anode and the cathode a layer sequence of a p-type emitter layer, an n-type base layer, a p-type base layer and an n-type emitter layer; and PA0 (c) on the side of the cathode a gate-cathode structure. PA0 (d) an n-doped intermediate layer is arranged between the n-type base layer and the p-type base layer; and PA0 (e) the doping concentration in the intermediate layer is higher than in the n-type base layer.
A component of this type is known as an MOS-controlled thyristor (MCT), for example, from the article by V. A. K. Temple, IEEE Transactions on Electron Devices, Vol. ED-33, No. 10 (1986), P.1609-1618, or as a gate turn-off thyristor (GTO), for example, from EP-A2-No. 0 241 662.
2. Discussion of Background
Nowadays modern power electronics demands fast semiconductor components controllable by simple means up to the highest power ranges. For low-end and medium power, hitherto power MOSFETs have proved particularly suitable owing to their large input impedance at the control electrode.
However, as is known, higher switchable powers can only be achieved with bipolar structures (power BJT, thyristors). Nevertheless, it is considered to be desirable to stick to the simple, and particularly low-power, control as is used with the power MOSFETs.
The positive aspects of the bipolar and MOS structures were first combined in the form of the IGBT (insulated gate bipolar transistor). The result of this combination was, as expected, components of the 1 kV class with substantially smaller forward resistances than the comparable power MOSFETs.
On the other hand, with IGBTs the inherent flooding, common to all bipolar components, of high-resistance area of the component is noticeable; of necessity they do not reach the high switching speeds of the corresponding unipolar components.
Today, controllable semiconductor components for the highest powers are the GTO (gate turn-off) thyristors. In principle, the entire chip surface is divided here too into a number of elementary cells connected in parallel. These components can be turned on and off via the gate contact; in this arrangement, however, an enormous outlay in terms of circuitry must be taken into account for the gate control.
A promising remedy here is MOS technology: various structures for such MCTs (MOS-controlled thyristors) can be specified, which all have cathode-side short-circuits switchable via MOS gates.
It has already been demonstrated in experiments that this principle permits current densities of more than 1000 Acm.sup.-2 to be turned off. With these MCTs, too, the structure of the cathode is similar to that of power MOSFETs and IGBTs. Thus, the elementary cells also have comparable dimensions (typical values are 20-30 .mu.m). If a larger component (e.g. on a 4" wafer) is to be realized, problems rapidly arise with the production yield. Nevertheless, the MCT is currently one of the most interesting candidates for future power semiconductor components.
It is currently usual to eliminate the inherent instabilities of the thyristors during dynamic operation by external connection to passive components. This connection limits the rate of rise of the anode voltage (dV/dt) to the values to be adhered to for safe turn-off.
In the case of thyristors of the GTO type, extremely non-homogeneous current density distributions (filaments) can occur during the turn-off. The current then concentrates essentially on one cathode element and can lead rapidly to the destruction of the thyristor as a result of thermal overloading. This effect must therefore be taken into account by a special design of the component, as well as by suitable protective circuits.
In general, two prerequisites must be fulfilled in a physical system to permit the formation of filaments: firstly the system must have a positive feedback. In the case of thyristors this is automatically provided by the pnpn structure. The second prerequisite is the existence of fluctuations of one or more physical quantities.
Known and already described in this context is, for example, the current concentration under the comparatively wide control segments in the conventional GTO. It is produced by the limited shunt conductance of the p-base; this triggering mechanism does not occur in finely structured elements such as the MCT.
However, the deduction should not be made from this that the MCT could turn off filament-free. According to the current state of knowledge, it is thought that in the MCT the phenomenon of "dynamic avalanche" is a trigger for the filamentation during the turn-off (see, for example,: M. Stoisiek et al., IEEE International Electron Devices Meeting Techn. Dig. (1987), p.666-669).
During the turn-off, a finely structured component such as the MCT runs through time sections in which there are still considerable currents flowing from the p-type base layer to the n-type base layer with a space-charge zone already building up at the p-n junction. Due to the rapidly growing electrical field, holes now drift from the still flooded n-base with drift saturation speed through the space-charge zone to the cathode.
The density of these holes, which depends only on the external current, greatly influences the field-strength distribution over the space-charge zone. Owing to their positive charge, the holes create an effective increase of the doping of the n-type base layer, which is expressed in a field rise at the p-n junction from the p-type base layer to the n-type base layer. Depending on the amount of external current, this amount can become so large that the breakdown field strength valid under static conditions is exceeded. The term "dynamic avalanche" refers to just such an increased field rise through free holes as well as to the premature breakdown.
The increased field rise created by drifting holes can only be significant in cases where the n-type base substrate doping is approximately equal to or is even less than the hole concentration. Thus, under the conditions of drift saturation, a current density of approximately 100 Acm.sup.-2 is carried by a hole density of 6*10.sup.13 cm.sup.-3. This value indeed approaches the substrate dopings of 1*10.sup.13 cm.sup.-3 to 1*10.sup.14 cm.sup.-3 typical for thyristors and automatically leads to a great increased field rise during turn-off.